Commentary for Avodah Zarah 94:14

Ami refers to the second clause in the mishnah. For it to become prohibited he must plaster or cement inside the stone itself, not merely on its outside. It seems that what he is trying to say is that for something to be considered an idol, it must be an idol “on the inside.” Almost the same way we treat human beings—our identity is formed more by our inside, than by our external features.